The present invention relates to a coated diamond to which an SiC coating is applied, a method of manufacturing and a composite material thereof. In particular, the invention relates to a coated diamond with an enhanced density as well as a superior adherence compared with those that have been accomplished, a method of manufacturing and a composite material thereof.
Diamond has the highest hardness among terrestrial materials and thus has been employed as tool materials. As diamond also has a considerably high thermal conductivity, application to heat sink materials is expected. When diamond is used as tool and heat sink materials, the diamond is molded with metal, ceramic, resin or the like into one piece by means of powder metallurgy and accordingly a composite thereof is produced. On the other hand, when the temperature of diamond is increased in the air, oxidation of the diamond begins at approximately 650xc2x0 C. A further shortcoming of the diamond is transformation into graphite resultant from dissolution of the diamond in molten metal of Co, Ni, Fe and the like. Then, in order to prevent oxidation as well as chemical reaction between diamond and these materials and to enhance a bonding strength between diamond and metal, ceramic and resin, studies have been conducted on application of a coating layer onto a surface of diamond (Japanese Patent Laying-Open Nos. 5-105560, 6-263516 and 8-113774).
However, conventional techniques of forming a coating layer on a surface of diamond are insufficient in terms of optimization of density, nonuniform thickness, surface properties and coating film thickness which affect quality of the coating layer. In addition, the bonding strength between the coating film and diamond is also insufficient.
The present invention aims to provide a diamond with a coating film that is dense and superior in adherence to enable diamond to exhibit its excellent characteristics, a method of manufacturing the coated diamond and a composite material of the coated diamond.
A coated diamond according to the present invention includes a diamond and a film including SiC as a main component to coat the diamond (hereinafter referred to as SiC film). The SiC is substantially formed of xcex2-SiC. For the SiC film, a value of ratio I (220)/I (111) is at least 0.38 and at most 0.55, I (220) representing peak intensity of Miller index (220) of SiC and I (111) representing peak intensity of Miller index (111) thereof.
The xcex2-SiC is SiC which belongs to one of Nos. 29-1129, 29-1128 and 39-1196 of JCPDS (Joint Committee on Powder Diffraction Standards) card. The film including xcex2-SiC as a main component preferably has at least a partial region which is in a crystallographically identical orientation relationship with respect to adjacent diamond, since such a coating film is unlikely to separate. The crystallographically identical orientation relationship (epitaxial relation) can be determined by irradiating a region in the vicinity of the boundary between the diamond and the SiC film with X-rays or electron beam to take an X-ray diffraction image or electron diffraction image where diffraction patterns of the diamond and SiC film occur. The present invention includes a structure having an intermediate layer between the diamond and the SiC film for alleviating lattice distortion to achieve a regular orientational relation between the diamond and SiC film, i.e., the diamond and SiC film are not directly adjacent to each other.
The xcex2-SiC can be identified by means of X-ray diffraction to measure a lattice constant of SiC, which is approximately 4.36 angstrom. The value of ratio I (220)/I (111) can be calculated by determining peak intensity in X-ray diffraction of Miller indices (220) and (111). As shown by JCPDS card, the value of I (220)/I (111) is usually 0.35. The value of ratio can be set at 0.38 or higher to considerably improve adherence of the xcex2-SiC film to diamond. The ratio is more desirably 0.4 or higher. However, the adherence deteriorates when the ratio exceeds 0.55 and thus the value is desirably 0.55 or lower, more desirably 0.5 or lower.
When xcex2-SiC is of equi-axial grains, the coating film has an enhanced strength and thus is unlikely to separate. Further, toughness of the xcex2-SiC is combined with the property of equi-axial grains to make the coating film more unlikely to separate. Here, xe2x80x9cequi-axial grainsxe2x80x9d refers to the shape of polycrystal grains having no anisotropy, as compared with columnar grains and extended grains having anisotropy. In the coated diamond according to the present invention, carbon atoms in the SiC film are supplied substantially from diamond. As C is supplied from diamond, the surface of diamond is readily coated completely with the xcex2-SiC film which is dense.
The xcex2-SiC film of equi-axial grains is not porous and the film coats the surface of diamond densely and completely. Accordingly, the bonding strength between the coating film and diamond can be enhanced. Here, xe2x80x9cfilm densely coatsxe2x80x9d means that a uniform coating film covers diamond without leaving a non-coated part.
The SiC film of the coated diamond discussed above could have an Si ratio which is lower near the diamond and higher near the surface of the SiC film. On the contrary, the ratio of carbon atoms could be higher near the diamond and lower near the surface of the SiC film. The Si and carbon ratios in the SiC film can be determined by spectroscopic analysis or EPMA (Electron Probe X-ray Microanalyser) and wavelength dispersive type X-ray spectrometer. For quantitative analysis of a light element like carbon having atomic number 10 or smaller, multi-layer crystal is used as a spectrometer in a wavelength dispersive type spectrometer, the spectrometer is placed in vacuum, and a proportional counter is used as a detector. Alternatively, secondary ion mass spectrometry (SIMS) may be used.
The film containing SiC as a main component of the coated diamond according to the present invention preferably has a thickness of 0.001 to 0.3 xcexcm.
The coating film having the thickness mentioned above prevents the diamond with superior characteristics from deteriorating. Specifically, oxidation resistance can be enhanced and prevention is possible of reaction between the diamond and molten metal of Co, Ni, Fe and the like. The coating film includes SiC with the carbon supplied from the diamond. Therefore, it is difficult to form the coating film with its thickness exceeding 0.3 xcexcm. On the other hand, a smaller thickness less than 0.001 xcexcm lessens the effect of applying the coating. Accordingly, the coating film has a thickness of 0.001 to 0.3 xcexcm.
More preferably, the film containing SiC as a main component of the coated diamond according to the present invention has a thickness ranging from 0.005 to 0.1 xcexcm.
The film thickness of 0.005 xcexcm or greater enables oxidation resistance to be sufficient and superior characteristics of the diamond to be retained. On the other hand, the film thickness can be limited to 0.1 xcexcm or smaller to prevent occurrence of crack in the coating film due to difference in thermal expansion coefficient between the diamond and SiC and to facilitate maintenance of the epitaxial relation between xcex2-SiC and adjacent diamond. Resistance to separation is thus significantly improved.
The SiC of the coated diamond according to the present invention desirably has a mean grain diameter of at most 50 nm for example.
When the mean grain diameter of SiC is 50 nm or smaller, the SiC has an improved strength. The mean grain diameter of SiC is desirably at most 15 nm. The SiC with the mean grain diameter in this range can have a further enhanced strength.
Diamond of the coated diamond according to the present invention is of particulate for example and the mean particle diameter of the diamond is 0.1 to 100 xcexcm.
When the diamond is formed of minute particles with a mean particle diameter of 0.1 to 100 xcexcm, a dense and uniform SiC film is difficult to produce by conventional means. On the other hand, the structure of the present invention enables a dense and uniform xcex2-SiC film to be formed on the surface of diamond of minute particles in a relatively easy manner. Then, with various characteristics of the diamond further enhanced, the SiC-coated diamond can be produced. It is noted that the coated diamond produced by applying a coating onto a powder material for powder metallurgy or the like is the particulate diamond discussed above.
It can be expected that a coated diamond produced by applying a coating onto a single crystal diamond of a certain shape like a plate has an excellent thermal conductivity. When the diamond is polycrystal, a manufacturing method of diamond such as vapor phase synthesis can be employed. Then, a plate-shaped material with a large area size can be prepared at a low cost.
With the structure described above, diamond of a certain shape like a plate is coated. It is accordingly possible to derive superior characteristics of the diamond by using the diamond for electronics materials such as heat sink materials.
The particulate diamond coated with the xcex2-SiC film can be employed as a material for powder metallurgy. Using this coated diamond particles, robust tool materials, electronics materials and the like having an arbitrary shape can be produced with superior diamond characteristics. Here, xe2x80x9cparticulate diamondxe2x80x9d refers to diamond particles and can be referred to as diamond powder alternatively. This relation between particulate and powder is also applied to other materials. The particulate diamond may be single-crystal particles or polycrystal particles. In other words, if the diamond is in the form of powder and can be used for powder metallurgy and the like, particles may be of single crystal or polycrystal.
A composite material according to the present invention includes at least one of metal, ceramic and resin, a diamond, and a xcex2-SiC film coating the diamond, and the material of at least one of metal, ceramic and resin and the diamond coated with that film are connected. In order to sufficiently derive excellent characteristics of the diamond, the SiC film desirably has a thickness of 0.005 to 0.1 xcexcm for example.
The diamond coated with the film having the thickness described above can be used as one component of a composite material to utilize excellent characteristics of the diamond which is firmly incorporated into the composite material. Then, there is a less deterioration of the diamond characteristics. Further, the SiC film is dense and superior in strength and has an excellent adherence to the diamond. Additionally, the SiC film itself has an excellent thermal conductivity so that a composite material to which the diamond is applied enables the diamond to sufficiently exhibit the superior characteristics. When the composite material is formed in one piece through powder metallurgy, the composite material is desirably formed of at least one of particulate metal, particulate ceramic and particulate resin and particulate diamond coated with the coating film explained above.
The particulate material, i.e., powder material can be used to form tool materials or electronics materials into one piece to produce a composite through powder metallurgy. Further, when at least one of metal of at least two kinds, ceramic of at least two kinds and resin of at least two kinds can be included to produce a composite material having an optimum composition based on optimum chemical components according to use.
When a composite material is produced by using the diamond coated with SiC according to the present invention and X-ray diffraction is applied directly to the composite material, calculation of a value of I (220)/I (111) of the SiC coating film could be impossible due to a weak peak intensity of the SiC. In this case, by means of a chemical method using acid or alkaline solution or thermal and mechanical method, the SiC only or the SiC-coated diamond can be obtained from the composite material and X-ray diffraction can be applied to thus obtained material.
According to a method of manufacturing a coated diamond of the present invention, silicon monoxide in solid phase and diamond in solid phase are arranged at respective positions having no contact with each other and allowing passage of gas, and the solid phase silicon monoxide and the solid phase diamond are heated in a temperature range of 1150 to 1450xc2x0 C.
According this manufacturing method, a chemical reaction of SiO (gas phase)+2C (diamond)xe2x86x92SiC (solid phase)+CO (gas phase) occurs, and carbon supplied from the diamond reacts. As the diamond to be coated is utilized as a source of carbon of SiC, the entire surface of the diamond is coated with a dense and solid xcex2-SiC film. As a conventional technique, formation of SiC through direct reaction between silicon powder and diamond has been proposed (Japanese Patent Laying-Open No. 8-113774). According to the proposed technique, an SiC film is formed through solid phase reaction. A resultant problem is that the SiC is locally formed and thus the diamond surface is likely to become rough. According to the present invention, the SiC film is formed through reaction via gas phase, therefore, diamond can be coated densely and uniformly with the SiC film.
The solid phase silicon monoxide is desirably of particles with a mean particle diameter of 0.1 to 1000 xcexcm for enhancement of density of gas phase SiO. When the mean particle diameter of particulate SiO exceeds 1000 xcexcm, vaporization of SiO does not immediately proceed. Then, it is likely that diamond has any part which is not covered. On the other hand, a mean particle diameter of SiO powder that is less than 0.1 xcexcm results in a high cost and troublesome handling. A more desirable range of the mean particle diameter of SiO powder is 0.5 to 500 xcexcm.
According to the method of manufacturing a coated diamond of the invention, the solid phase silicon monoxide is arranged below the solid phase diamond and a porous separation layer is placed between the silicon monoxide and the diamond.
This structure achieves an arrangement of the silicon monoxide and diamond in a simple manner, the arrangement allowing passage of gas phase while preventing contact between the silicon monoxide and diamond. SiO gas generated from solid phase silicon monoxide moves upward and can contribute to reaction for forming a coating layer on the diamond surface without loss since the diamond is arranged in the close vicinity thereof.
According to the method of manufacturing a coated diamond of the invention, the porous separation layer is a felt formed of carbon.
The carbon felt is formed of the same carbon as that of the diamond and the porous property is stably maintained to allow passage of SiO gas without contacting the silicon monoxide and diamond. In this way, the carbon felt of low cost can be employed to contribute to reaction for forming a coating layer on the diamond surface without loss of generated SiO gas. It is noted that the carbon felt which is used once by the method of the present invention for coating the diamond surface with SiC and accordingly has SiC attached to the surface can thereafter be used continuously for forming an SiC film on diamond without loss of SiO gas. Such a continued use of the carbon felt can contribute to formation of an SiC film on diamond without further loss of SiO gas.
According to the method of manufacturing a coated diamond of the present invention, the silicon monoxide and diamond are placed in a lidded container, the container provided in a high vacuum of 10xe2x88x922 Torr or higher, and thereafter subjected to heating.
With this structure discussed above, the reaction for forming the coating layer on the diamond surface can be proceeded with a less oxidation loss and the like and an increased partial pressure of the gas phase SiO.
According to the method of manufacturing a coated diamond of the present invention, the container is an alumina crucible having its interior covered with a carbon sheet. A lid formed of alumina is put on the alumina crucible to shield contents of the alumina crucible from outside air, and the alumina crucible including its contents is heated for 10 minutes to 6 hours.
The manufacturing method described above makes it possible to efficiently form a dense xcex2-SiC film using an apparatus at hand without preparing a special crucible or the like.
According to the method of manufacturing a coated diamond, the diamond and the silicon monoxide placed in the alumina crucible satisfy a weight ratio of (2:1) to (20:1).
According to the method of manufacturing a coated diamond of the invention, the weight ratio in this range enables an SiC film of a uniform thickness to be formed on the diamond surface.
The diamond with its weight smaller than the one represented by (2:1) produces a local part with a greater film thickness. On the other hand, the weight increased to exceed the one represented by (20:1) produces a part of diamond that is not covered with the SiC film.
This diamond may be single crystal diamond with a mean particle diameter of 0.1 to 3000 xcexcm or single crystal and plate-shaped diamond. When the mean particle diameter of particulate diamond exceeds 3000 xcexcm, the diamond is too large as material for powder metallurgy so that a one-piece formed product with a great strength cannot be manufactured. On the other hand, when the mean particle diameter of diamond particles is less than 0.1 xcexcm, the ratio of SiC in the coating film to diamond is too large. Then, superior diamond characteristics cannot be derived as the diamond formed into one-piece. The diamond particles have a mean particle diameter which is more desirably 0.5 xcexcm to 1500 xcexcm.
According to the method of manufacturing a coated diamond of the invention, the diamond may be polycrystal and in the shape of a plate for example.
The plate-shaped diamond can be coated with the film of a high thermal conductivity including SiC as a main component to apply the diamond to various electronics materials like heat sink materials.